2 Answers
2

Unless there is an answer in the director's commentary to the film (which I have heard but don't recall), the only decent answer may very well be @SSumner's comment to your question: "because Peter Jackson." But it would seem that Tolkien's description of the immediate aftermath of Sauron's defeat in the book might have provided sufficient inspiration for the more visually arresting scene of the ground opening and swallowing the armies of Mordor:

As when death smites the swollen brooding thing that inhabits their crawling hill and holds them all in sway, ants will wander witless and purposeless and then feebly die, so the creatures of Sauron, orc or troll or beast spell-enslaved, ran hither and thither mindless; and some slew themselves, or cast themselves in pits, or fled wailing back to hide in holes and dark lightless places far from hope.

RotK, Book VI, Ch. 4, The Field of Cormallen, p. 227 (HM 2nd Revised ed. 1965). While this could be considered speculation, Jackson likely viewed his representation of the scene as more dramatic, and more easily depicted on screen, than Tolkien's description of some orcs doing one thing, some orcs doing another, and some (not quoted above but in the text) desperately fighting to the death.

While there was a physical battle between orcs and men, there was also a battle of wills between Sauron and the Istari and Elves (this is pretty evident from the books - particularly where Galadriel and Gandalf [and Elrond?] are revealed to be ring-bearers).

Sauron dying meant that whatever his powers were sustaining would have been easily affected by the powers of those opposing him.

It's worth noting that this sundering of the ground didn't happen in the original book.

This is the first time I've heard about a 'battle of wills' in this situation, could you provide a source, or a description how you come to this conclusion?
–
cfrei89Jul 9 '14 at 5:31

incorrect answer. While there are vague references of a battle of minds, it's explicitly stated that the powers of Galadriel, Elrond, and Gandalf (the wearers of the Three) are greatly diminished by the destruction of the One.
–
jwentingJul 9 '14 at 13:24

Diminished is qualitative - there's no benchmarks for the powers of the Elven-ring bearers before or after. There's also no evidence that the diminishment was immediate.
–
HorusKolJul 9 '14 at 22:50

@cfrei89 - there's not a direct reference - but it is alluded to: there's some kind of balance between the Elven-rings and the One ring (they were all forged to Sauron's pattern); Galadriel is (barely) holding back Sauron to protect Lothlorien, and Elrond the same for Rivendell; and there's a subtext to the sections with Saruman.
–
HorusKolJul 9 '14 at 22:54